This particular event started July 6, 2017 and will end on July 24, 2017 1 p.m. PDT.
There are been many special events before. Click this link to view past events
Yeah, Pokemon GO is sort of rough on phones. It's a battery drainer through and through. If applicable, use a phone case that has an external battery. For example, I use the Apple Smart Case with my iPhone, basically giving me ~200% battery life.
The Shiny Magikarp was limited? I had heard that some Raids against Magikarp are more likely to be shiny. If this isn't the case, I have wasted a handful of Raid passes.